Phase diagram of soybean phosphatidylcholine-diacylglycerol-water studied by x-ray diffraction and 31P- and pulsed field gradient 1H-NMR: evidence for reversed micelles in the cubic phase

Phase behavior of soybean phosphatidylcholine and glycerol dioleate in hydrated and dehydrated states studied by small-angle X-ray scattering

Soybean phosphatidylcholine (SPC) and glycerol dioleate (GDO) form liquid crystal nanostructures in aqueous environments, and their mixtures can effectively encapsulate active pharmaceutical ingredients (API). When used in a subcutaneous environment, the liquid crystalline matrix gradually hydrates and degrades in the tissue whilst slowly releasing the API. Hydration dependent SPC/GDO phase behavior is complex, non-trivial, and still not fully understood. A deeper understanding of this system is important for controlling its function in drug delivery applications. The phase behavior of the mixture of SPC/GDO/water was studied as a function of hydration and lipid ratio. Small-angle X-ray scattering (SAXS) was used to identify space groups in liquid crystalline phases and to get detailed structural information on the isotropic reverse micellar phase. The reported pseudo ternary phase diagram includes eight different phases and numerous multiphase regions in a thermodynamically consistent way. For mixtures with SPC as the predominant component, the system presents a reverse hexagonal, lamellar and R3m phase. For mixtures with lower SPC concentrations, reverse cubic (Fd3m and Pm3n) as well as intermediate and isotropic micellar phases were identified. By modeling the SAXS data using a core-shell approach, the properties of the isotropic micellar phase were studied in detail as a function of concentration. Moreover, SAXS analysis of other phases revealed new structural features in relation to lipid-water interactions. The new improved ternary phase diagram offers valuable insight into the complex phase behavior of the SPC/GDO system. The detailed structural information is important for understanding what APIs can be incorporated in the liquid crystal structure.

NMR Studies of Bicontinuous Liquid Crystalline Phases of Cubic Symmetry: Interpretation of Frequency-Dependent Relaxation Rates

Extensive deuterium NMR relaxation data are presented for two specifically deuterium labeled surfactants forming bicontinuous cubic phases with water. ²H spin-lattice (R₁) and spin-spin (R₂) relaxation rates were measured over an extended frequency range from 2 to 60 MHz. The data are interpreted with an existing theoretical framework for spin relaxation in bicontinuous cubic phases, which takes its starting point in the description of bicontinuous phases using periodic minimal surfaces. We show that the theory succeeds in accounting for the data and that the defining parameters of the theory, correlation times and order parameters, are in agreement with related data in other surfactant phase situations. Specifically, we obtain the surfactant self-diffusion coefficient over the minimal surface in one unit cell and show that it is in agreement with the corresponding macroscopic NMR diffusion data. By measuring two additional NMR relaxation parameters for each carbon on the surfactant hydrocarbon tail, we demonstrate how order parameter and correlation time profiles can be obtained. Finally, we analyze published molecular dynamics trajectories for a bicontinuous cubic phase. The analysis provides further support for the theoretical framework used to interpret relaxation data.

Fast understanding of phases and phase separation in liquid crystal drug delivery systems using deuterium solid-state NMR

Characterization of lipid based (SPC/GDO/H₂O) liquid crystal (LC) drug delivery system is non-trivial and highly complex, especially when multiple and intermediate phases are present. The phase behavior of such mixtures during hydration or delivery is still poorly understood and therefore, characterizing these systems is crucially important towards controlling their function and enhancing the understanding of their drug release behavior. Current work has established an easy way to identify liquid crystal phases and phase mixtures using deuterium (²H) solid-state nuclear magnetic (NMR) spectroscopy under static conditions without disrupting the three dimensional structure and phases, as magic-angle spinning (MAS) could lead to disruption of the phases. Small angle X-ray scattering (SAXS) technique and optical microscopy were also employed to corroborate the study.

Linking (Pyr)1apelin-13 pharmacokinetics to efficacy: Stabilization and measurement of a high clearance peptide in rodents

Apelin, the endogenous ligand for the APJ receptor, has generated interest due to its beneficial effects on the cardiovascular system. Synthesized as a 77 amino acid preproprotein, apelin is post-translationally cleaved to a series of shorter peptides. Though (Pyr)¹apelin-13 represents the major circulating form in plasma, it is highly susceptible to proteolytic degradation and has an extremely short half-life, making it challenging to quantify. Literature reports of apelin levels in rodents have historically been determined with commercial ELISA kits which suffer from a lack of selectivity, recognizing a range of active and inactive isoforms of apelin peptide. (Pyr)¹apelin-13 has demonstrated beneficial hemodynamic effects in humans, and we wished to evaluate if similar effects could be measured in pre-clinical models. Despite development of a highly selective LC/MS/MS method, in rodent studies where (Pyr)¹apelin-13 was administered exogenously the peptide was not detectable until a detailed stabilization protocol was implemented during blood collection. Further, the inherent high clearance of (Pyr)¹apelin-13 required an extended release delivery system to enable chronic dosing. The ability to deliver sustained doses and stabilize (Pyr)¹apelin-13 in plasma allowed us to demonstrate for the first time the link between systemic concentration of apelin and its pharmacological effects in animal models.

The lipolytic degradation of highly structured cubic micellar nanoparticles of soy phosphatidylcholine and glycerol dioleate by phospholipase A2 and triacylglycerol lipase

The effects of different lipolytic enzymes on the structure of lipid liquid crystalline nano-particles (LCNP) have been investigated by cryogenic transmission electron microscopy (cryo-TEM) and synchrotron small angle X-ray diffraction (SAXD). Here we used highly structured cubic micellar (Fd3m) nanoparticles of 50/50 (wt%/wt%) soy phosphatidyl choline (SPC)/glycerol dioleate (GDO) as substrate. Two types of lipolytic enzymes were used, phospholipase A2 (PLA2) that catalyses degradation of the phospholipid component, SPC, and porcine pancreatic triacylglycerol lipase (TGL) that facilitate the hydrolysis of the diglyceride, GDO. Evolution of the structure was found to be very different and linked to specificity of the two types of enzymes. PLA2, which hydrolyses the lamellar forming component, SPC, induces a reversed micellar lipid phase, while TGL which hydrolysis the reverse phase forming compound, GDO, induces a lamellar phase.

Complex three-dimensional self-assembly in proxies for atmospheric aerosols

Aerosols are significant to the Earth’s climate, with nearly all atmospheric aerosols containing organic compounds that often contain both hydrophilic and hydrophobic parts. However, the nature of how these compounds are arranged within an aerosol droplet remains unknown. Here we demonstrate that fatty acids in proxies for atmospheric aerosols self-assemble into highly ordered three-dimensional nanostructures that may have implications for environmentally important processes. Acoustically trapped droplets of oleic acid/sodium oleate mixtures in sodium chloride solution are analysed by simultaneous synchrotron small-angle X-ray scattering and Raman spectroscopy in a controlled gas-phase environment. We demonstrate that the droplets contained crystal-like lyotropic phases including hexagonal and cubic close-packed arrangements of spherical and cylindrical micelles, and stacks of bilayers, whose structures responded to atmospherically relevant humidity changes and chemical reactions. Further experiments show that self-assembly reduces the rate of the reaction of the fatty acid with ozone, and that lyotropic-phase formation also occurs in more complex mixtures more closely resembling compositions of atmospheric aerosols. We suggest that lyotropic-phase formation likely occurs in the atmosphere, with potential implications for radiative forcing, residence times and other aerosol characteristics.

Nearly all atmospheric aerosols contain surface-active organic compounds; however, the nature of how they arrange remains poorly understood. Here, the authors show that fatty acids in atmospheric aerosol proxies self-assemble into highly ordered, viscous 3D nanostructures that undergo changes upon exposure to humidity and ozone.

Non-lamellar lipid liquid crystalline structures at interfaces

The self-assembly of lipids leads to the formation of a rich variety of nano-structures, not only restricted to lipid bilayers, but also encompassing non-lamellar liquid crystalline structures, such as cubic, hexagonal, and sponge phases. These non-lamellar phases have been increasingly recognized as important for living systems, both in terms of providing compartmentalization and as regulators of biological activity. Consequently, they are of great interest for their potential as delivery systems in pharmaceutical, food and cosmetic applications. The compartmentalizing nature of these phases features mono- or bicontinuous networks of both hydrophilic and hydrophobic domains. To utilize these non-lamellar liquid crystalline structures in biomedical devices for analyses and drug delivery, it is crucial to understand how they interact with and respond to different types of interfaces. Such non-lamellar interfacial layers can be used to entrap functional biomolecules that respond to lipid curvature as well as the confinement. It is also important to understand the structural changes of deposited lipid in relation to the corresponding bulk dispersions. They can be controlled by changing the lipid composition or by introducing components that can alter the curvature or by deposition on nano-structured surface, e.g. vertical nano-wire arrays. Progress in the area of liquid crystalline lipid based nanoparticles opens up new possibilities for the preparation of well-defined surface films with well-defined nano-structures. This review will focus on recent progress in the formation of non-lamellar dispersions and their interfacial properties at the solid/liquid and biologically relevant interfaces.

Phospholipid-based nonlamellar mesophases for delivery systems: bridging the gap between empirical and rational design

Phospholipids are ubiquitous cell membrane components and relatively well-accepted ingredients due to their natural origin. Phosphatidylcholine (PC) in particular offers a promising alternative to monoglycerides for lyotropic liquid crystalline (LLC) delivery system applications in the food, cosmetics and pharmaceutical industries, provided its strong tendency to form zero-mean curvature lamellar mesophases in water can be overcome. Higher negative curvatures are usually reached through the addition of a third lipid component, forming a ternary diagram phospholipid/water/oil. The initial part of this work summarizes the potential advantages and the challenges of phospholipid-based delivery system applications. In the next part, various ternary PC/water/oil systems are discussed, with a special emphasis on the PC/water/cyclohexane and PC/water/α-tocopherol systems. We report that R-(+)-limonene has a quantitatively similar effect as cyclohexane. The last part is devoted to the theoretical interpretation of the observed phase behaviors. A fruitful parallel is drawn with PC polymer-like reverse micelles, leading to a thermodynamic description in terms of interfacial bending energy. Investigations at the molecular level are reviewed to help in bridging the empirical and theoretical approaches. Predictive rules are finally derived from this wide-ranging overview, thereby opening the way to a future rational design of PC-based LLC delivery systems.

Formation of highly structured cubic micellar lipid nanoparticles of soy phosphatidylcholine and glycerol dioleate and their degradation by triacylglycerol lipase

Lipid nanoparticles of reversed internal phase structures, such as cubic micellar (I2) structure show good drug loading ability of peptides and proteins as well as some small molecules. Due to their controllable small size and inner morphology, such nanoparticles are suitable for drug delivery using several different administration routes, including intravenous, intramuscular, and subcutaneous injection. A very interesting system in this regard, is the two component soy phosphatidylcholine (SPC)/glycerol dioleate (GDO) system, which depending on the ratio of the lipid components form a range of reversed liquid crystalline phases. For a 50/50 (w/w) ratio in excess water, these lipids have been shown to form a reversed cubic micellar (I2) phase of the Fd3m structure. Here, we demonstrate that this SPC/GDO phase, in the presence of small quantities (5-10 wt %) of Polysorbate 80 (P80), can be dispersed into nanoparticles, still with well-defined Fd3m structure. The resulting nanoparticle dispersion has a narrow size distribution and exhibit good long-term stability. In pharmaceutical applications, biodegradation pathways of the drug delivery vehicles and their components are important considerations. In the second part of the study we show how the structure of the particles evolves during exposure to a triacylglycerol lipase (TGL) under physiological-like temperature and pH. TGL catalyzes the lipolytic degradation of acylglycerides, such as GDO, to monoglycerides, glycerol, and free fatty acids. During the degradation, the interior phase of the particles is shown to undergo continuous phase transitions from the reversed I2 structure to structures of less negative curvature (2D hexagonal, bicontinuous cubic, and sponge), ultimately resulting in the formation of multilamellar vesicles.

Bioadhesive lipid compositions: self-assembly structures, functionality, and medical applications

Lipid-based liquid crystalline compositions of phospholipids and diglycerides have unique bioadhesive properties with several medical applications, as exemplified by a lipid-based medical device indicated for management and relief of intraoral pain. The present paper describes the relation between self-assembly properties of phosphatidyl choline (PC) and glycerol dioleate (GDO) mixtures in the presence of aqueous fluids and functional attributes of the system, including: film formation and bioadhesion, intraoral coverage, acceptance by patients, and potential as a drug delivery system. The phase behavior of PC/GDO was characterized using synchrotron small-angle X-ray scattering. Functional properties, including the presence of study formulations at intraoral surfaces, ease of attachment, taste, and degree of and intraoral pain, were assessed in a crossover clinical pilot study in head and neck cancer patients. An optimum in functional properties was indicated for formulations with a PC/GDO weight ratio of about 35/65, where the lipids form a reversed cubic liquid crystalline micellar phase structure (Fd3m space group) over the relevant temperature range (25-40 °C).

Conical lipids in flat bilayers induce packing defects similar to that induced by positive curvature

In biological membranes, changes in lipid composition or mechanical deformations produce defects in the geometrical arrangement of lipids, thus allowing the adsorption of certain peripheral proteins. Here, we perform molecular dynamics simulations on bilayers containing a cylindrical lipid (PC) and a conical lipid (DOG). Profiles of atomic density and lateral pressure across the bilayer show differences in the acyl chain region due to deeper partitioning of DOG compared to PC. However, such analyses are less informative for the interfacial region where peripheral proteins adsorb. To circumvent this limitation, we develop, to our knowledge, a new method of membrane surface analysis. This method allows the identification of chemical defects, where hydrocarbon chains are accessible to the solvent, and geometrical defects, i.e., voids deeper than the glycerol backbone. The size and number of both types of defects increase with the number of monounsaturated acyl chains in PC and with the introduction of DOG, although the defects do not colocalize with the conical lipid. Interestingly, the size and probability of the defects promoted by DOG resemble those induced by positive curvature, thus explaining why conical lipids and positive curvature can both drive the adsorption of peripheral proteins that use hydrophobic residues as membrane anchors.

Adsorption of lipid liquid crystalline nanoparticles: effects of particle composition, internal structure, and phase behavior

Controlling the interfacial behavior and properties of lipid liquid crystalline nanoparticles (LCNPs) at surfaces is essential for their application for preparing functional surface coatings as well as understanding some aspects of their properties as drug delivery vehicles. Here we have studied a LCNP system formed by mixing soy phosphatidylcholine (SPC), forming liquid crystalline lamellar structures in excess water, and glycerol dioleate (GDO), forming reversed structures, dispersed into nanoparticle with the surfactant polysorbate 80 (P80) as stabilizer. LCNP particle properties were controlled by using different ratios of the lipid building blocks as well as different concentrations of the surfactant P80. The LCNP size, internal structure, morphology, and charge were characterized by dynamic light scattering (DLS), synchrotron small-ange X-ray scattering (SAXS), cryo-transmission electron microscopy (cryo-TEM), and zeta potential measurements, respectively. With increasing SPC to GDO ratio in the interval from 35:65 to 60:40, the bulk lipid phase structure goes from reversed cubic micellar phase with Fd3m space group to reversed hexagonal phase. Adding P80 results in a successive shift toward more disorganized lamellar type of structures. This is also seen from cryo-TEM images for the LCNPs, where higher P80 ratios results in more extended lamellar layers surrounding the inner, more dense, lipid-rich particle core with nonlamellar structure. When put in contact with a solid silica surface, the LCNPs adsorb to form multilayer structures with a surface excess and thickness values that increase strongly with the content of P80 and decreases with increasing SPC:GDO ratio. This is reflected in both the adsorption rate and steady-state values, indicating that the driving force for adsorption is largely governed by attractive interactions between poly(ethylene oxide) (PEO) units of the P80 stabilizer and the silica surface. On cationic surface, i.e., silica modified with 3-aminopropltriethoxysilane (APTES), the slightly negatively charged LCNPs give rise to a very significant adsorption, which is relatively independent of LCNP composition. Finally, the dynamic thickness measurements indicate that direct adsorption of intact particles occurred on the cationic surface, while a slow buildup of the layer thickness with time is seen for the weakly interacting systems.

Adsorption of lipid liquid crystalline nanoparticles on cationic, hydrophilic, and hydrophobic surfaces

Investigation of nonlamellar nanoparticles formed by dispersion of self-assembled lipid liquid crystalline phases is stimulated by their many potential applications in science and technology; resulting from their unique solubilizing, encapsulating, and space-dividing nature. Understanding the interfacial behavior of lipid liquid crystalline nanoparticles (LCNPs) at surfaces can facilitate the exploitation of such systems for a number of potentially interesting uses, including preparation of functional surface coatings and uses as carriers of biologically active substances. We have studied the adsorption of LCNP, based on phosphatidylcholine/glycerol dioleate and Polysorbate 80 as stabilizers, at different model surfaces by use of in situ ellipsometry. The technique allows time-resolved monitoring of the layer thickness and the amount adsorbed, thereby providing insights into the restructuring of the lipid nanoparticle upon adsorption. The effects of solvent condition, electrolyte concentration, particle size, and surface chemistry on adsorbed layer properties were investigated. Furthermore, the internal structures of the particles were investigated by cryo-transmission electron microscopy and small angle X-ray diffraction on the corresponding liquid crystalline phases in excess water. LCNPs are shown to form well-defined layers at the solid-liquid interface with a structure and coverage that are determined by the interplay between the self-assembly properties of the lipids and lipid surface interactions, respectively. At the hydrophobic surface, hydrophobic interaction results in a structural transition from the original LCNP morphology to a monolayer structure at the interface. In contrast, at cationic and hydrophilic surfaces, relaxation is a relatively slow process, resulting in much thicker adsorbed layers, with thickness and adsorption behavior that to a greater extent reflect the original bulk LCNP properties.

A characterisation study on the application of inverted lyotropic phases for subcutaneous drug release

An experimental characterisation of lipid mixtures consisting of inverted hexagonal and inverted cubic phases composed of soybean phosphatidylcholine (SPC) and glycerol dioleate (GDO) was performed. The release of five chromophores of varying lipophilicity, used as model drugs, was investigated. Two experimental setups were applied: one based on maintaining sink condition, while a constant volume release medium was employed for the other. For neither setup, no correlation between the model drug lipophilicity and the polarity of the carrier matrix was found. However, the lipid phases showed a prolonged release, spanning weeks, of the model drugs, which exhibit lipophilicity values ranging by four orders of magnitude.

High-resolution NMR characterization of a gel-like surfactant mesophase

The addition of phosphatidylcholine to AOT water-in-oil microemulsions leads to the formation of a rigid gel as the water content is increased above a specific threshold. This system is a gel-like crystalline phase where the microstructure evolves from reverse hexagonal to lamellar with increasing water content and/or temperature. Couette sheared (1)H and (31)P NMR experiments carried out at varying temperature and water content show distinct signatures with microstructure evolution. Because the system has been fully characterized through small-angle neutron scattering, it is possible to relate the NMR signatures to the microstructure. The NMR technique therefore complements scattering techniques but is additionally useful because the technique also picks up isotropic signatures from concurrently occurring noncrystalline phases. The use of NMR to identify such lyotropic gel-like crystalline phases allows easy correlation between templated materials synthesis in these phases and phase microstructure. NMR can therefore be used as a probe to understand microstructure in specific surfactant systems and to characterize the retention of microstructure during materials synthesis.

Structural and rheological investigation of Fd3m inverse micellar cubic phases

In the present study we demonstrate that a bulk inverse micellar cubic phase of Fd3m structure can be obtained by adding a hydrophobic component, such as the food-grade limonene, to the binary system monolinolein/water in a well-defined composition. The Fd3m structure studied in this work had a very slow kinetics of formation, as a consequence of partitioning of water into two types of micelle populations with different sizes. The Fd3m structure formed at a ratio of limonene oil to total lipids of alpha = 0.4 is stable in the bulk up to a maximum hydration of 12.68 wt % water, beyond which it starts to coexist with dispersed water. At full hydration, by combining small-angle X-ray scattering and available topological models, the inverse micellar cubic phase of Fd3m structure was shown to be formed by 16 small micelles and 8 larger micelles per cubic lattice cell (Q227 group), with radii of the micellar polar cores ranging between 1 and 3 nm and 149-168 monolinolein molecules per micelle depending on the water content. The temperature dependence of the structural and rheological properties of the Fd3m mesophase was investigated using SAXS, rheology, and turbidimetry. It appeared that the Fd3m phase underwent crystallization below 18 degrees C and began melting in an inverse microemulsion (L2 phase) coexisting with water above 28.5 degrees C with complete melting obtained at 40-45 degrees C, as evidenced by SAXS and rheology. Macroscopic phase separation between the L2 phase and excess water was observed with time at higher temperatures. The investigation of the viscoelastic properties of the Fd3m inverse discrete micellar cubic phase revealed a rheological signature similar to that of the bicontinuous cubic phases Pn3m and Ia3d observed in homologous binary systems. However, the Fd3m phase presented a complex set of slower relaxation mechanisms leading to a shift by 1 order of magnitude of the dominant relaxation times and whole relaxation spectrum, as compared to those of inverse bicontinuous cubic phases. These findings have been tentatively explained by (i) the multiple relaxation of micelles upon deformation, (ii) the small hydration level of the Fd3m phase, and (iii) the low temperature at which this phase can be observed.

The "inverted cup" -- a novel in vitro release technique for drugs in lipid formulations

The aim of this study was to develop a membrane-free in vitro release method for drugs in lipid formulations. It was intended to be applicable to as wide a range as possible of preparations, independently of their polarity and viscosity. The principle of the novel technique is to keep the sample suspended in the release medium in an inverted glass cup, allowing a possible phase transition or swelling. Thirteen formulations containing bupivacaine, lidocaine and/or prilocaine in lipid vehicles with different physical properties were prepared and examined. When possible, in vitro release profiles obtained by the new method were compared to profiles obtained by earlier techniques. For three formulations of either bupivacaine or lidocaine in polar lipid formulations, in vitro release profiles were evaluated in relation to in vivo data, from nerve block and pharmacokinetic studies in rats. Preparations that could be investigated both by the "inverted cup" and by the earlier published "single drop" technique generally showed good agreement between the two release profiles. In the case of the polar lipid formulations, arterial blood concentration curves in rats could reasonably be predicted from the in vitro release profiles. In conclusion, the "inverted cup" technique should potentially be applicable to a wide range of lipid formulations of drugs, both for physico-chemical characterisation and for obtaining in vitro -- in vivo correlations.

31P and 1h NMR as probes of domain alignment in a rigid crystalline surfactant mesophase

A viscous reverse hexagonal surfactant mesophase containing bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and alpha-phosphatidylcholine (lecithin), with comparable volume fractions of isooctane and water, was characterized by Fourier transform (31)P and (1)H NMR spectroscopy. Shear alignment was reflected through both (31)P NMR and (1)H NMR spectra. A complicated (31)P spectrum was observed as a result of superposition of chemical shifts according to the distribution of crystalline domains prior to shear. The initially disordered samples with polydomain structures become macroscopically aligned after Couette shear. (31)P NMR chemical shift anisotropy characteristics are used to elucidate orientation of the hexagonal phase. Interestingly, (1)H NMR spectra exhibit spectral changes upon shear alignment closely corresponding with that of (31)P NMR spectra. These observations complement the findings of mesophase alignment obtained using SANS and imply that (31)P and (1)H NMR spectroscopy can be used as probes to define microstructure and monitor orientation changes in this binary surfactant system. This is especially beneficial if these mesophases are used as templates for materials synthesis.

Lamellar-to-cubic phase change in phospholipid bilayer systems incorporated with block copolymers: DMPC and PEO-PPO-PEO (P85)

We have used small-angle X-ray scattering and calorimetric methods to investigate the temperature-dependent phase behavior of ternary systems of phospholipid (DMPC), amphiphilic PEO-PPO-PEO block copolymer (Pluronics P85), and water. It is shown that a relatively small amount of block copolymers ( approximately 3 wt %) results in a lamellar-to-cubic phase transition. Still, both the bilayer-characteristic main transition, associated with chain melting, and the pretransition, associated with in-plane modulations, are preserved for copolymer concentrations up to 50-70 wt %, indicating the preservation of a bilayer type of lipid organization also within the cubic phase. The main transition splits up into two transitions upon the addition of copolymers, one resembling the high cooperativity of the main transition and one broad transition which may reflect complex formation with the copolymers. Parallel studies incorporating poly(ethylene glycol) into the DMPC multilamellar vesicles do not give analogous structural changes. It is concluded that the major effect on the molecular scale of adding PEO-PPO-PEO block copolymers is not only due to the hydration of the membrane but also due to the incorporation of the PPO block into the bilayer structure.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Inclusion of lignocaine base into a polar lipid formulation--in vitro release, duration of peripheral nerve block and arterial blood concentrations in the rat

BACKGROUND

Slow-release formulations of local anaesthetics may produce nerve blocks of long duration. The present study aimed at investigating the in vitro and in vivo properties of a polar lipid formulation for slow release of lignocaine and the effects on nerve block duration by inclusion of dexamethasone into the system.

METHODS

In vitro release of lignocaine from the lipid formulation was studied in a US Pharmacopoeia rotating apparatus. Sciatic nerve blocks were induced in rats by 0.1 ml of test formulations containing lignocaine HCl 20 mg. ml-1 in aqueous solution, lignocaine base 20, 100 or 200 mg. ml-1 in lipid formulation or the last formulation with dexamethasone 0.05, 0.5 or 5 mg. ml-1. The durations of sensory and motor block and the arterial blood concentrations of lignocaine were investigated.

RESULTS

In vitro there was a sustained release of lignocaine from the lipid formulation, with 50% release at around 48 h. In vivo lignocaine base 20 mg. ml-1 in lipid formulation produced sciatic nerve blocks of significantly shorter duration than lignocaine HCl 20 mg. ml-1 in aqueous solution, while lignocaine base 100 and 200 mg. ml-1 in lipid formulation produced blocks lasting two and three times longer, respectively, than the lignocaine HCl solution. Addition of dexamethasone did not affect the duration of nerve block. Following administration of lignocaine base 200 mg. ml-1 in lipid formulation, as compared to lignocaine HCl 20 mg. ml-1 in aqueous solution, the maximal blood concentration of lignocaine was only three times higher in spite of the ten-fold difference in dose, and the mean terminal half-life was three times longer, reflecting the slow release from the formulation.

CONCLUSIONS

Our findings indicate that lignocaine base in polar lipids acts as a slow-release preparation of local anaesthetic both in vitro and in vivo.

Curvature properties of novel forms of phosphatidylcholine with branched acyl chains

We studied the properties of a series of phosphatidylcholine molecules with branched acyl chains. These lipids have previously been shown to have marked stimulatory effects on the side-chain cleavage activity of cytochrome P450SCC (CYP11A1), an enzyme of the inner mitochondrial membrane. The synthetic lipids used were diacyl phosphatidylcholines with the decanoyl, dodecanoyl or tetradecanoyl chain having a hexyl, octyl or decyl straight chain aliphatic branch at the 2-position. All three lipids lowered the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine, the lipids with longer acyl chains being more effective in this regard. As pure lipids all of the forms were found by X-ray diffraction to be predominantly in the hexagonal phase (HII) over the entire temperature range of 7-75 degrees C. The properties of the HII phase were unusual with regard to the small size of the lattice spacings and the small temperature dependence of the spacings. We used tetradecane to relieve hydrocarbon packing constraints to determine the intrinsic radius of curvature of the lipid monolayer. The elastic bending modulus was measured in the presence of tetradecane by introducing an osmotic gradient across the hexagonal phase cylinders with aqueous solutions of poly(ethylene glycol). The elastic bending modulus was found to be higher than that observed with other lipids and to increase with temperature. Both the small intrinsic radius of curvature and the high elastic bending modulus indicate that the presence of these lipids in bilayer membranes will impose a high degree of negative curvature strain.

Hydroxyapatite Granule/Carrier Composites Promote New Bone Formation in Cortical Defects

BACKGROUND

A great deal of interest has been focused on finding substitutes for autogenous bone grafts. Among the most interesting materials are different calcium phosphate compositions (e.g., hydroxyapatite [HA]), due to their biocompatible properties in hard and soft tissue.

PURPOSE

The bone response to porous ceramic HA granules in combination with two lipid and one polysaccharide carrier was evaluated in an experimental bone defect model in rabbits.

MATERIALS AND METHODS

Circular defects (Ø 4 mm) were made in both tibias of 32 rabbits. The 64 defects were divided into four groups. Group A was augmented with a composite of HA granules and a phospholipid-diacetyl-glycerol carrier, group B with HA granules and a phospholipid carrier, group C received HA granules and a sodium hyaluronan carrier, and group D served as control. The animals were killed after 6 weeks and ground sections were evaluated using light microscopic morphometry. X-ray microfluorescence (XRF) was applied in order to evaluate the suitability of this method to examine bone-biomaterial interfaces. Calcium distribution was studied using x-ray fluorescence line scans at selected interface regions of two sections in group B.

RESULTS

The HA/phospholipid composites were easier to shape and handle than the HA/hyaluronan composite. Group A had 36% newly formed bone area within the defect. Groups B and C showed significantly more newly formed bone within the defect (47% and 49%, respectively) compared to the control group (31%). The XRF analysis revealed that the amount of calcium in the newly formed bone was similar to that observed for the HA granules and slightly lower when compared to the mature, lamellar bone.

CONCLUSIONS

Synchrotron radiation may be a new, suitable technique to study the interface between bone and biomaterials with regard to mineral content. The results suggest that HA granule/lipid and HA granule/hyaluronan composites have interesting properties as bone-substitute materials.

Orientational behavior of phospholipid membranes with mastoparan studied by 31P solid state NMR

Solid state 31P NMR spectroscopy was used to study the perturbing effect of the wasp venom peptide mastoparan (MP) on lipid bilayers composed of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG). The 31P chemical shift anisotropy of multilamellar vesicles decreased with increasing peptide concentration, indicating that MP interacts strongly and selectively with the charged DMPG head group. Macroscopically oriented MP-lipid samples between glass plates were studied by 31P NMR as a function of tilt angle. These spectra showed the coexistence of orientation-dependent lamellar signals as well as an isotropic peak, suggesting that MP can induce non-lamellar phases in DMPC/DMPG membranes.

Phase Equilibria and Structure of the 1-Dodecylpyridinium Bromide-Dodecane-Water System

The isothermal ternary phase diagram for the 1-dodecylpyridinium bromide/dodecane/water system was determined at 40 degreesC by 2H NMR and polarizing microscopy methods. Two liquid crystalline phases, a large cubic area and a normal hexagonal phase, and one isotropic normal micellar solution phase were characterized, and their ranges of existence were determined. The micelles were found to be probably small and spherical at lower concentrations of surfactant, and were found to grow at higher concentrations and on addition of oil. The two-phase areas, L1 + HI and HI + I, are both very narrow. The comparatively large cubic area, containing 43-63 wt% surfactant and 3-10 wt% dodecane, is probably consistent of more than one structure. SAXS experiments indicate two different structures built of discrete micellar aggregates. Copyright 1998 Academic Press.

Morphological changes induced by phospholipase C and by sphingomyelinase on large unilamellar vesicles: a cryo-transmission electron microscopy study of liposome fusion

Cryo-transmission electron microscopy has been applied to the study of the changes induced by phospholipase C on large unilamellar vesicles containing phosphatidylcholine, as well as to the action of sphingomyelinase on vesicles containing sphingomyelin. In both cases vesicle aggregation occurs as the earliest detectable phenomenon; later, each system behaves differently. Phospholipase C induces vesicle fusion through an intermediate consisting of aggregated and closely packed vesicles (the "honeycomb structure") that finally transforms into large spherical vesicles. The same honeycomb structure is also observed in the absence of enzyme when diacylglycerols are mixed with the other lipids in organic solution, before hydration. In this case the sample then evolves toward a cubic phase. The fact that the same honeycomb intermediate can lead to vesicle fusion (with enzyme-generated diacylglycerol) or to a cubic phase (when diacylglycerol is premixed with the lipids) is taken in support of the hypothesis according to which a highly curved lipid structure ("stalk") would act as a structural intermediate in membrane fusion. Sphingomyelinase produces complete leakage of vesicle aqueous contents and an increase in size of about one-third of the vesicles. A mechanism of vesicle opening and reassembling is proposed in this case.

Cryo-TEM and NMR studies of a micelle-forming phosphoglucolipid from membranes of Acholeplasma laidlawii A and B

The chemical structure of a phosphoglucolipid from the membrane of the bacterium Acholeplasma laidlawii strain B-PG9 has been determined by high resolution NMR to be 1,2-diacyl-3-O-[glycerophosphoryl-6-O-(alpha-D-glucopyranosyl-(1 -->2)-O-alpha-D-glucopyranosyl)]-sn-glycerol (GPDGlcDAG). It was concluded that this lipid has exactly the same structure as one of the phosphoglucolipids from A. laidlawii strain A-EF22. By cryo transmission electron microscopy (cryo-TEM) and NMR diffusion techniques it was shown that, in highly diluted aqueous solutions, this membrane lipid forms long thread-like micelles in equilibrium with lipid vesicles. The cause of the occurrence of these different aggregates is discussed in terms of the varying molecular shapes of the lipid because of a heterogeneous composition of the acyl chains. A second membrane phosphoglucolipid from the bacterium, namely 1,2-diacyl-3-O-[glycerophosphoryl-6-O-(alpha-D- glucopyranosyl-(1 -->2)-monoacylglycerophosphoryl-6-O-alpha-D-glucopyranosyl)]-sn-gl ycerol (MABGPDGlcDAG), was found to form only a lamellar liquid crystalline phase coexisting with water.

Model of interaction between a cardiotoxin and dimyristoylphosphatidic acid bilayers determined by solid-state 31P NMR spectroscopy

The interaction of cardiotoxin IIa, a small basic protein extracted from Naja mossambica mossambica venom, with dimyristoylphosphatidic acid (DMPA) membranes has been investigated by solid-state 31P nuclear magnetic resonance spectroscopy. Both the spectral lineshapes and transverse relaxation time values have been measured as a function of temperature for different lipid-to-protein molar ratios. The results indicate that the interaction of cardiotoxin with DMPA gives rise to the complete disappearance of the bilayer structure at a lipid-to-protein molar ratio of 5:1. However, a coexistence of the lamellar and isotropic phases is observed at higher lipid contents. In addition, the number of phospholipids interacting with cardiotoxin increases from about 5 at room temperature to approximately 15 at temperatures above the phase transition of the pure lipid. The isotropic structure appears to be a hydrophobic complex similar to an inverted micellar phase that can be extracted by a hydrophobic solvent. At a lipid-to-protein molar ratio of 40:1, the isotropic structure disappears at high temperature to give rise to a second anisotropic phase, which is most likely associated with the incorporation of the hydrophobic complex inside the bilayer.